Rated 1 out of 5 stars

I don’t understand why one would want to include 4-5 dictionaries in one. It makes no sense but the main problem is that once installed, these dictionaries reset themselves as your default every time you restart Firefox! So, start your browser, type something in it and every English word is underlined for French corrections because this app turned off you English dictionary to turn itself on. This issue hasn’t been fixed in several updates despite unanimous criticism over this choice of programming.

Currently, the best workaround is this:
You need to delete the “reform” dictionary from the app folder which is located here:
C:\Users\YOURUSERNAME\AppData\Roaming\Mozilla\Firefox\Profiles\6181lk90.default\extensions\fr-dicollecte@dictionaries.addons.mozilla.org\dictionaries

(You can copy this link into a Windows Explorer window and just change "YOURUSERNAME" to yours.)

You can delete other dictionaries but the files you HAVE to get rid of are “fr-reform.dic” and “fr-reform.aff”. That’s the one that resets itself as the default language.
Now, turn off automatic updates for this thing in your Add-On window. If you let the app updates itself, it'll start changing your default language again.

This review is for a previous version of the add-on (5.0.2). 

Firefox only remembers the last choice of dictionary you made. This extension DOES NOT modify your settings and HAS NO priority on other extensions or dictionaries. This extension just extends the dictionaries choice.

I never saw this issue on my system (Windows 7 with the latest Firefox). I often switch between French and English and Firefox just remembers the last setting.

I assume this problem occurs on corrupted profile or on Linux distros which usually create big mess among dictionaries.